Measurement of cardiac output is a basic diagnostic and therapeutic requirement in the management of patients with cardiac conditions. There are three widely accepted methods which accurately measure cardiac output, and they are therefore recognized as standard methods and serve as gold-standard methods for the evaluation of new technologies for cardiac output measurement. These include the direct Fick's principles based parameters (the rate of diffusion is proportional to the difference in concentration, accordingly, the volume of oxygen consumed per unit time is proportional to the difference in oxygen content between arterial and venous blood, with the degree of proportionality depending on the volume of blood pumped per unit time), dye dilution, and bolus thermodilution. All these methods measure the rate of dilution of a known volume indicator. The indicator for Fick is oxygen, thermodilution uses cold saline or dextrose, dye dilution uses cardiogreen, and they are all invasive techniques that require specialist expertise and expensive capital equipment, can be employed only on hospital premises, and all pose a certain risk.
The development of non-invasive alternative methods is of great practical value.
Currently, two technologies are accepted in clinical settings for non-invasive measurement of cardiac output. These are Doppler Echocardiography and Impedance Cardiography methods. Although the Doppler echocardiography technique is reliable, both techniques require expensive equipment and specialist expertise.
Three basic technologies exist for measuring cardiac output by electrical bioimpedance:
(A) Thoracic Impedance Cardiography (TIC).
According to this technique, electrodes are applied to the root of the neck and to the lower part of the thorax, and the chest is measured as an electrical field. The method was first suggested in 1964 [Patterson et al., Proc. 1st Annu Rocky Mt. Bioeng. Sympos., 1964, 56-71] and was upgraded in 1974 [Kubicek et al. Biomed. Eng. 1974, 9:410-16] where the ΔR parameter was replaced by its first derivative, the dR/dt, of the impedance change, multiplied by the ejection time (T), for the stroke volume calculation. Subsequently, a number of variations in the variables of the formula were offered, but the TIC cardiac output results remained inaccurate in patients with cardiac conditions (Handelsman H, Health Technology Assessment Reports, US Dept Health and Human Services, Public Agency for Health Care Policy and Research 1991; 6:1-13; Raaijmakers et al, Ann NY Acad Sci 1999; 873:121-34).
The sources of errors in TIC established by experimental and clinical data include the following: (1) individual anatomical differences in the position of organs in the thorax cage; (2) non-linear changes of the electrical resistivity of the flowing blood; (3) the current electrodes position and the distance between the voltage electrodes; (4) the ejection pattern; (5) the outflow problem; (6) the influences of multifactors on the thoracic basal impedance value; (7) the inability to assess the contribution from the right heart and pulmonary circulation; and (8) the inability to assess impedance changes independently of volume changes, such as mechanical displacements of the heart and great vessels.
(B) Whole Body ICG (ICGWB)
According to this technique, tetrapolar electrodes are applied to all four limbs. The method was suggested in 1973 by Tischenko [Sechenov Phisiological J 1973; 59:1216-24)], and is currently promoted by Koobi et al., Crit Care Med 1999; 27:2206-11.
In many conditions, the accuracy of the ICGWB technique is higher than that of the TIC due to the fact that the peripheral systolic impedance changes are more reliable signals for calculating the stroke volume than the thoracic systolic impedance waveforms. Nevertheless, this technology is cumbersome, and in many clinical conditions the extremities are used for invasive IV or IA procedures such as intra-arterial pressure monitoring, IV treatment, etc. Movement artifacts also increase due to the use of multiple electrodes. All these factors make it difficult or impractical to use these methods in clinical conditions.
(C) Regional Impedance Cardiography (RIC)
This technique utilizes two pairs of electrodes, one pair of electrodes being applied to one wrist and the other pair of electrodes—to the contralateral ankle. As an electrical field between the electrodes sites is a body region which consists of three segments—one arm, the trunk, and one leg, this method is called regional impedance cardiography (RIC). This technique was approbated in cardiac surgery clinics in 1998 by Cohen et al. [Eur J Cardiothorac Surg 1998; 14:64-9]; Cotter et al., Accurate, noninvasive continuous monitoring of cardiac output by whole-body electrical bioimpedance, Chest. 2004 April; 125(4): 1431-40; Cotter et al, Impedance cardiography revisited. Physiol. Meas., 2006, 27:817-827. Compared to TIC, the accuracy of RIC is higher by a factor of 2.